1. Field of the Invention
This invention relates to a method for preparing a flexible magnetic recording medium.
2. Related Background Art
Pursuit of higher densification in magnetic recording has brought about high coercive force, thinner film of magnetic layer, etc. in magnetic recording medium. However, vertical magnetic recording has been proposed as the result of a pursuit of higher densification according to the method different from this, and practical applications have been progressed in various fields. The constitution of a vertical magnetic recording medium having the electromagnetic converting characteristics most approximate to practical application known in the art is so called flexible magnetic disc, comprising a magnetic layer of CoCr alloy thin film formed on a flexible polymeric substrate.
Such constitution had the drawback which was essentially inevitable during formation of the magnetic layer that the medium is subject to curved deformation, namely generation of curl, due to internal stress of thin film. When curl is generated, no good contact can be obtained between the magnetic head and the medium, thereby inconveniences occur such as lowering in reproduction output, lowering in S/N ratio or instability in running, etc. In realizing practical application, prevention of curl is extremely important.
One of the methods practiced in the prior art for prevention of curl of flexible magnetic disc is formation of magnetic layers on both surfaces, and another method is formation of a magnetic layer only on one surface to utilize heat shrinkage of the substrate, or provision of a coated layer on the back surface to cancel curl, but formation of magnetic layers on both surfaces is more suitable for practical application in view of simplicity of the steps.
As practical preparation steps, as shown schematically in FIG. 1, there has been generally known the method in which magnetic layers are formed on the polymeric substrate by continuous sputtering or continuous vapor deposition while a lengthy polymeric substrate 4 is moved continuously. Also, there has been used the system in which the substrate is moved as flat without use of the rotatory drum as shown in FIG. 2. Here, the rotatory drum 3 or the substrate holder 7 is adapted to be elevatable to desired temperature when forming a magnetic layer on the polymeric substrate for improvement of the magnetic characteristics of the CoCr magnetic layer. As the polymeric substrate, polyethyleneterephthalate, polyimide, polyamide, etc. have been known as representative ones.
However, according to such method of the prior art in which magnetic layers are formed on both surfaces of the polymeric substrates by use of polyethyleneterephthalate, polyimide, polyamide, etc., and the magnetic characteristics are improved by heating treatment during formation, the medium obtained suffered from curls generated irregularly in the longer direction of the medium, namely including the portion where curls can be removed and the portion where curls are generated, whereby curls cannot be removed stably. For this reason, there has been the problem that the production yield is low.
On the other side, a thermal treatment of a substrate before formation of a magnetic layer is proposed, for example, in Japanese Patent Laid-Open Application Nos. 69-139139, 60-121532, 60-129931, 60-133540 and 60-191434. However, conditions in a thermal treatment a polyimide substrate are not well known.
The use of polyimide as a substrate of a magnetic recording medium is described, for example, in Japanese Patent Laid-Open Application Nos. 60-138722 and 62-215631.